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Thursday, March 19, 2015

Is a Heat Detector Required for the Elevator Pit

Designers are always asking, "Do I need to put a heat detector in the elevator pit?"

"Are you required to install a heat detector in the bottom of the elevator shaft otherwise known as the elevator pit?".  This is a question that comes up a lot in the fire alarm industry and often has system designers and AHJs (Authority Having Jurisdiction) scratching their heads.  Another related questions is, "Why is there a sprinkler head located at the bottom of the elevator shaft?".  A sprinkler head located in the bottom of the elevator pit is in place to control the spread of fire caused by the ignition of trash and debris that has fallen through the door opening and collected over time.

Heat Detector in Elevator PitThere are two items that need to be present before the requirement of a fire alarm system heat detector is required.  One is the presence of an automatic sprinkler head.  NFPA 13 2010 ed. 8.15.5 states that sprinklers heads are to be installed in the top and bottom of the elevator shaft.  There are exceptions to this rule so keep in mind that not all elevator shafts will incorporate a sprinkler head.  Two is the height in which the sprinkler head is installed off the floor of the elevator pit.  ASME A17.1 states that if a sprinkler head is installed within 24" (2 feet) of the elevator pit floor, it shall be exempt from the special arrangements of inhibiting water flow until the elevator recall function has occurred.

A heat detector is required to be installed within 2' of any sprinkler head associated with shutting down the power to an elevator (NFPA 72 2010 ed. 21.4.2*). It is important to shut down the elevator power prior to the release of water from a sprinkler head since water and electronics do not mix.  This is the reason the heat detector is required to be set to a lower temperature setting and higher sensitivity setting than the sprinkler head (NFPA 72 2010 ed 21.4.1*). With that said, a heat detector is not required if the sprinkler head is located within 24" of the elevator pit floor since there is typically not any electrical components located in this area.

There are three common methods to shutting down the main elevator power prior to water flowing from a sprinkler head in the shaft or elevator machine room.

#1) The most economical method is to use a waterflow switch.  Upon activation, the waterflow switch would cause an alarm at the FACU (Fire Alarm Control Unit) as well as activate the shunt trip breaker causing the power to be interrupted.  Make sure you follow NFPA 72 2010 ed 21.4.3*.  This code section states that if using waterflow or pressure switches to shut down elevator power, the use of a time delay shall not be permitted.

#2) This is the most common method.  By use of a fixed temperature rate of rise heat detector located within 2' of each sprinkler head in the shaft, hoistway or elevator machine room.  The heat detector shall be set to a lower temperature than the sprinkler head and when activated, will cause an alarm at the FACU and shunt the breaker associated with powering the elevator.

#3) Use of a pre-action system.  These systems would have supplemental fire detection devices installed in the same areas as the sprinkler heads.  Make note that the detection devices should be heat detectors.  Once on of the heat detectors have been activated, it would tell the pre-action control panel through program mapping to open a valve control by a solenoid.  Once the valve is open, water would then fill the sprinkler system piping in the elevator hoistway and elevator equipment room.  At the same time, the heat detector would also trip the shunt breaker thus shutting down the elevator power.  If a fire really is present in these areas, it would eventually fuse the sprinkler head and release water to the affected area.

Keep in  mind that heat detectors are to be used for shutting down power to the elevator and not smoke detectors.  Smoke detectors associated with elevators are to be solely used for designated elevator recall, alternate elevator recall and to activate the elevator hoistway smoke relief equipment.